Method and Device for Compressing a Gaseous Medium

ABSTRACT

A method and device for compressing a gaseous medium, specifically hydrogen, is disclosed. Compression of the gaseous medium takes place by way of a fluid, where a fluid is used in which the gaseous medium is not soluble and/or can be separated residue-free from the gaseous medium. An ionic fluid, a high-boiling hydraulic oil, or a fluid which has a very low vapor pressure is used as the fluid.

This application claims the priority of International Application No. PCT/EP2005/008370, filed Aug. 2, 2005, and German Patent Document No. 10 2004 046 316.6, filed Sep. 24, 2004, the disclosures of which are expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for compressing a gaseous medium, specifically hydrogen.

The invention further relates to a device for compressing a gaseous medium, specifically hydrogen.

In methods and devices of a generic kind for compressing a gaseous medium, reciprocating piston compressors or reciprocating piston compressor systems are normally used at the present time. Reciprocating piston compressors require appropriate sealing systems in order to keep the medium to be compressed separate from the medium driving the piston, for example hydraulic oil.

Particularly in the compression of hydrogen, natural gas and high purity media, or if contamination of the medium to be compressed by the drive medium must be prevented and/or is undesirable for specific reasons, precisely fitting cylinders with pistons and correspondingly effective dynamic sealing systems are required; as a rule, these systems result in high production and maintenance costs. Often, even more cost-intensive compression variants, such as diaphragm compressors, oil-free reciprocating piston compressors, etc., are brought in for such applications.

Generic methods and devices of a generic kind are used, for example, in natural gas compressor stations such as are found in natural gas filling stations.

The object of the present invention is to specify a method and a device of a generic kind for compressing a gaseous medium, specifically hydrogen, which avoid the aforementioned disadvantages.

Concerning the method, this object is achieved through the compression of the gaseous medium by a fluid, wherein a fluid is used in which the gaseous medium is not soluble and/or which can be separated residue-free.

The characteristics of the device in accordance with the invention for compressing a gaseous medium are that it comprises

a) one or more cylinders,

b) supply and discharge lines which serve to supply the gaseous medium to be compressed to or remove it from the cylinder or cylinders,

c) at least one fluid line per cylinder which serves to supply and remove the fluid compressing the gaseous medium in the cylinders, and

d) means to change the quantity of fluid in the cylinder or cylinders,

e) wherein the fluid is a fluid in which the gaseous medium to be compressed is not soluble and/or which can be separated residue-free from the gaseous medium.

The invention makes it possible to dispense with a piston and any (piston) sealing systems in the compression of a gaseous medium. This is managed by achieving compression of the gaseous medium to be compressed by way of a variable column of fluid inside a cylinder. The pistons used previously, which consist of a solid material, are replaced by a incompressible fluid, or column of fluid. Through an upward and downward movement of the column of fluid—similar to the upward and downward motion of a piston—the gaseous medium to be compressed is drawn in and compressed.

In order to achieve optimal compression even of high purity media which must not be contaminated by the fluid being used, a fluid is preferably selected in which the gaseous medium to be compressed is not soluble and which can be separated residue-free from the gaseous medium.

In an advantageous way, an ionic fluid, a high-boiling hydraulic oil or fluids which have a very low vapor pressure, as for example, vacuum pump oils, molten salts and metals with a low melting point, or fluids which have a gas solubility of less than 10⁻⁴ mol/l bar are used as fluid.

Ionic fluids are low-boiling, organic salts with melting points between 100 and −90° C., where most of the known ionic fluids are already present in liquid form at room temperature. In contrast to conventional molecular fluids, ionic fluids are completely ionic and thus reveal new and unusual properties. Ionic fluids are comparatively easily adaptable in their properties to given technical problems as a result of the variation in the structure of anion and/or cation and the variation in their combinations. For this reason they are frequently also described as “designer solvents.” With conventional molecular fluids on the other hand, only a variation in the structure is possible.

In contrast to conventional molecular fluids, ionic fluids have the additional advantage that they possess no measurable vapor pressure. This means that—as long as their decomposition temperature is not reached—they do not boil off to the slightest degree, even in a total vacuum. From this result their properties of non-flammability and environmental friendliness since, as a result, ionic fluids cannot reach the atmosphere.

As already mentioned, the melting points of known ionic fluids are by definition below 100° C. The liquidus range—the range between melting point and thermal decomposition—is usually 400° C. or higher.

In addition, ionic fluids have very high thermal stability. Their decomposition points are frequently above 400° C. In the case of ionic fluids, their density and mixing characteristics with other fluids can be affected, or adjusted, through the choice of ions. Ionic fluids have the additional advantage that they are electrically conductive and as a result can prevent static electrical charges—which represent a potential hazard.

Ionic fluids have the advantage that it is possible to separate them completely from the compressed medium with a comparatively small expenditure for equipment.

Entertainment of the ionic fluid by the compressed medium is henceforth not possible since ionic fluids—as mentioned previously—have no vapor pressure.

In the case of fluids with high gas solubility, there is firstly undesirable cavitation of the drive pump(s) and secondly undesirable entrainment of gas into the (interim) fluid storage tank which is normally provided. Through the use of a fluid which has a gas solubility of less than 10⁻⁴ mol/l bar, these problems can be avoided. As a result, the life of the drive pump used is extended; further, the safety-related problems accompanying the gas formation, or entrainment, are avoided.

The method in accordance with the invention, the device in accordance with the invention and further embodiments of same are explained in more detail using the embodiment shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows a potential embodiment of the invention in which compression takes place in two separate cylinders Z1 and Z2. Alternatively, compression can be carried out in only one or also in more than two cylinders.

DETAILED DESCRIPTION OF THE DRAWING

The gaseous medium to be compressed is brought to cylinders Z1 and Z2 through the lines 1, 1′ and 1″. Inlet valves a and b are located in the aforementioned lines. After compression has taken place, the compressed gaseous medium is drawn off from cylinders Z1 and Z2 through the discharge lines 2′ and 2″ in which valves c and d are similarly located.

The compressed gaseous medium is freed in a separating device A of any fluid which may have been entrained from cylinders Z1 and Z2, and which will be looked at more closely in what follows, and is then taken by way of line 2 to be used further and/or to interim storage.

A suitable fluid D is provided inside cylinders Z1 and Z2 which serves to compress the gaseous medium. Cylinders Z1 and Z2 are connected by way of lines 3 to 6 and hydraulic pump X which is driven by an electric motor M.

The fluid levels D in the cylinders Z1 and Z2 are varied by means of the hydraulic pump X such that one of the cylinders draws in the medium to be compressed while simultaneously, or essentially simultaneously, the gaseous medium is compressed in the other cylinder. Preferably an axial piston pump with swash plate drive is used for this, where transfer volume and/or direction can be changed through a simple adjustment of the swash plate.

Compared with the prior art, the invention has the further advantage that the (compression) heat created during compression can be removed at least partially by way of the fluid D. For this, as shown in the drawing, heat exchangers, or radiators, K1 and K2, are provided through which the heat created in the cylinders during compression can be discharged to the environment and/or another suitable medium. In the case of complete removal of compression heat through the fluid D and the heat exchangers, or radiators, K1 and K2, isothermal, single-stage compression can be realized.

Valves e or g are located between the radiators K1 and K2 and the hydraulic pump X; the effect of these so-called stationary valves is that no system pressure is present at the hydraulic pump X when it is not running.

In accordance with an advantageous embodiment of the device in accordance with the invention, heat exchangers E1 or E2 can be located in the cylinders Z1 and Z2.

In the compressor or cylinder designs reckoned among the prior art, cooling of the cylinder chamber can only be implemented from outside since the moving piston inside the cylinder does not permit the provision of a heat exchanger. Until now, the heat generated during compression has therefore been given off by the compressor or cylinder outer jacket to the cooling medium (air, water, coolant, etc.). Because of this fact, compression cannot normally be carried out isothermally, which results in corresponding high compression energy.

By means of the aforementioned advantageous embodiment of the device in accordance with the invention, internal cooling can now be implemented, the consequence of which is that the disadvantages of the prior art can be avoided.

The term “heat exchanger” is understood to mean any designs for heat exchangers—designated as “active heat exchanger” in what follows—and thermal reservoirs—designated as “passive heat exchanger” in what follows.

While the heat arising during compression is removed by means of a suitable cooling medium in the case of an active heat exchanger, this heat remains inside the compressor or cylinder chamber in the case of a passive heat exchanger. In the latter case, the compression heat is in fact extracted from the medium to be compressed, but is then given off to the fluid D which carries away the compression heat—as explained above. Cooling ribs, fins, etc., and/or fillers such as metal spheres, plates, etc., can be used as passive heat exchangers, or thermal reservoirs respectively.

The aforementioned advantageous embodiment of the device in accordance with the invention allows a substantial reduction in the required compression energy and thereby approximately isothermal compression. Furthermore, lower gas exit temperatures can be realized, and a reduction in the thermal load on the compressor valves can be achieved.

The fluid coming from the cylinders Z1 and Z2 and separated from the compressed medium in the separating device A is taken by way of line 9, in which a shutoff valve i is located, to an optionally provided interim storage tank S. From here, the fluid can be taken according to need by way of the lines 7 and 8 and the two shutoff valves f and h to the cylinders Z1 and/or Z2.

In accordance with a further advantageous embodiment of the method in accordance with the invention for compressing a gaseous medium, the replenishing of fluid required for compression takes place during a suction stroke.

There is a loss of fluid D particularly at the drive, or hydraulic, pump needed for compression. In order to compensate for these losses, it is therefore necessary to replenish or fill the system with new fluid when fluid falls below a minimum level. Care must be taken that there is no interaction between the pressure and suction sides while fluid is being filled. Furthermore, care must be taken that the desired or maximum energy requirement of the system, which must be defined by the compression process, is not (unnecessarily) increased.

The embodiment of the method for compressing a gaseous medium described previously creates an opportunity for filling fluid in which the requirements described previously can be satisfied.

As a matter of course, the time for replenishing fluid required for compression should be guided by the current power consumption of the system; preferably fluid replenishment should take place during or close to a power minimum. At this time, the system, or the drive pump, has sufficient power reserves available which can be used for replenishing the fluid.

Preferably the fluid D to be replenished is fed into the appropriate cylinder, Z1 or Z2 respectively, by way of line 10, which has a feed pump P, during a suction stroke. However, care must be taken that replenishment does not take place in the immediate proximity of the reversal point since then the danger exists that fluid D might possibly escape from the corresponding cylinder Z1 or Z2 by way of the pressure line 2′ or 2″. The consequence would be that the separation device A which serves to separate fluid entrained from the cylinders Z1 and Z2 would have to be dimensioned correspondingly larger. Adding fluid during the suction stroke also minimizes the energy requirements of the feed pump P.

Fluid loss is detected by measuring the deviations of the fluid levels in cylinders Z1 and Z2 from a reference value which is usually determined at the beginning of the compression process.

In accordance with a further advantageous embodiment of the method for compressing a gaseous medium in accordance with the invention, the fluid is exposed to an electrical field. For this purpose, means for generating an electrical field in the cylinder or cylinders are to be provided on the device side.

Particularly when ionic fluids come into direct contact with other media (gases, fluids, etc.), the result can be intermixing at the interface and formation of a two-phase mixture. In the present case, such a two-phase mixture can arise, for example, inside the cylinder or cylinders at the interface between ionic fluids and the medium to be compressed.

For a clean and reliable separation of ionic fluid and the medium to be compressed, an adequate difference in density and a matching gravitational field—which in the present case is generated by the acceleration of gravity—are required. The maximum reversal acceleration inside the cylinder is defined thereby. In the Earth's gravitational field, the consequence is that a maximum acceleration of 7 m/s² can be realized. Acceleration of this nature is, however, frequently not sufficient to reseparate completely the two-phase mixture that has resulted.

By means of the embodiments of the method in accordance with the invention described previously, or the device in accordance with the invention, natural force fields such as gravitation, Coreolis force, etc., can be intensified by an electrical field.

These embodiments can be realized with all ionic fluids which have a corresponding dipole moment and/or corresponding electrical conductivity.

Influencing ionic fluids by means of an electrical field allows an increase in acceleration at the reversal points of the pistonless compressor without increased risk of phase intermixing. Furthermore, a clean and reliable separation of ionic fluids from a two-phase mixture is also possible when the differences in density between the ionic fluid and the medium to be compressed are comparatively minor. In addition to the embodiment of the invention explained using the drawing, embodiments of the method in accordance with the invention and the device in accordance with the invention are realizable for which only one cylinder or three or more cylinders are provided. While continuous delivery of the compressed medium with respect to compression pressure is not possible with only one cylinder, such often desirable delivery of the compressed medium with two or more cylinders is possible.

Considerable savings in investment costs are achieved by dispensing with solid pistons and dynamic sealing systems. In addition, maintenance costs are reduced since the maintenance intervals, compared with those of conventional compressors, are extended.

The invention is suitable for compressing gaseous media up to currently attainable pressures of 1000 bar. It should be emphasized that in principle higher pressures of any type are attainable. The invention further makes possible compression to maximum pressure with only a single compression stage. Furthermore, the transfer volume can be varied as desired. Particularly with respect to the compression of high purity media, the invention creates an economical opportunity for compressing such media as well to very high pressures. 

1-16. (canceled)
 17. A method for compressing a gaseous medium, wherein a compression of the gaseous medium is carried out by a fluid, wherein the gaseous medium is not soluble in the fluid and/or the fluid is separable residue-free from the gaseous medium.
 18. The method according to claim 17, wherein the gaseous medium is hydrogen.
 19. The method according to claim 17, wherein an ionic fluid, a high-boiling hydraulic oil, a fluid which has very low vapor pressure, or a fluid which has a gas solubility of less than 10⁻⁴ mol/l bar, is used as the fluid.
 20. The method according to claim 17, wherein compression heat is at least partially removed by means of the fluid.
 21. The method according to claim 17, wherein a portion of the fluid, entrained by the gaseous medium when compressed, is separated from the compressed gaseous medium.
 22. The method according to claim 21, wherein the portion of the fluid separated from the compressed gaseous medium is returned again for the compression, wherein the separated fluid is storable before it is returned.
 23. The method according to claim 17, wherein the compression allows compression of the gaseous medium by a factor of
 1000. 24. The method according to claim 17, wherein a replenishment of fluid required for compression takes place during a suction stroke.
 25. The method according to claim 17, wherein the fluid is exposed to an electrical field.
 26. A device for compressing a gaseous medium, comprising: one or more cylinders; supply and discharge lines which supply the gaseous medium to be compressed to, and discharge a compressed gaseous medium from, the cylinder or cylinders, respectively; at least one fluid line per cylinder which supplies and discharges a fluid compressing the gaseous medium in the cylinder or cylinders; and means for changing a volume of the fluid in the cylinder or cylinders; wherein the gaseous medium is not soluble in the fluid and/or the fluid is separable residue-free from the gaseous medium.
 27. The device according to claim 26, wherein the gaseous medium is hydrogen.
 28. The device according to claim 26, wherein the means for changing the volume of the fluid in the cylinder or cylinders is a fluid pump, a hydraulic pump, an axial piston pump, a slide pump, or a gear pump.
 29. The device according to claim 26, wherein a heat exchanger which removes compression heat is assigned to the fluid line provided per cylinder.
 30. The device according to claim 26, wherein at least one separation device is located in the discharge line for the compressed gaseous medium, wherein the separation device separates a portion of the fluid, entrained with the compressed gaseous medium, from the compressed gaseous medium.
 31. The device according to claim 30, wherein the separation device is connected in a circulation loop to at least one of the cylinders.
 32. The device according to claim 30, wherein a fluid storage device is allocated to the separation device.
 33. The device according to claim 26, wherein a heat exchanger is located within the cylinder or cylinders.
 34. The device according to claim 26, wherein means to generate an electrical field in the cylinder or cylinders is provided.
 35. A method for compressing a gaseous medium, comprising the steps of: supplying the gaseous medium to a cylinder; supplying a fluid to the cylinder; and compressing the gaseous medium in the cylinder by the fluid in the cylinder.
 36. A device for compressing a gaseous medium, comprising: a cylinder; a gaseous medium supply line coupled to the cylinder, wherein the gaseous medium supply line supplies the gaseous medium to the cylinder; a fluid supply line coupled to the cylinder, wherein the fluid supply line supplies a fluid to the cylinder; wherein the gaseous medium supplied to the cylinder is compressed by the fluid supplied to the cylinder; and a compressed gaseous medium discharge line coupled to the cylinder, wherein the compressed gaseous medium discharge line discharges the compressed gaseous medium from the cylinder. 